The invention relates to a high strength multiphase steel.
The invention also relates to a method for producing a hot or cold rolled strip from such a steel according to patent claim 9.
The invention relates in particular to steels with tensile strengths in the range of from 580-900 MPa with low yield ultimate ratio of below 67% for producing components which have excellent formability and welding properties.
The hotly contested automobile market forces manufacturers to constantly seek solutions to lower the fleet consumption while at the same time maintaining a highest possible comfort and occupant protection. Hereby weight saving of all vehicle components plays an important role but also a best possible behavior of the individual components under conditions of high static or dynamic stress during use and also in the event of a crash. Pre material suppliers seek to account for this requirement by providing high-strength and ultra-high strength steels with thin sheet thickness to reduce the weight of the vehicle components while at the same time improving forming and component properties during manufacture and during use.
High-strength and ultra-high strength steels enable more lightweight vehicle components (for example passenger cars and trucks) which as a consequence leads to reduced fuel consumption. The reduced CO2 proportion associated therewith leads to a reduction in pollution.
These steels therefore have to meet relatively high demands regarding their strength and ductility, energy absorption capacity and during processing, such as for example during pickling, the hot or cold forming, welding and/or surface treatment (for example metallically finished, organically coated, varnishing).
Newly developed steels thus must met the demands on the required weight reduction, the increasing material demands on yield strength, tensile strength and elongation at break at good formability, as well as demands on the component of high tenacity, border crack resistance, energy absorption and strength via the work hardening effect and the bake hardening effect, but also improved suitability for joining in the form of improved weldability.
Improved edge crack resistance means increased hole expansion and is known under synonymous terms such as high hole expansion (HHE) or low edge crack (LEC).
Improved weldability is achieved inter alia by a lowered carbon equivalent. For this stand synonymous terms such as low carbon equivalent (LCE) or under peritectical (UP).
In vehicle construction dual phase steels are therefore increasingly used, which consist of a ferritic basic structure in which a martensitic second phase and possibly a further phase with bainite and residual austenite is integrated. Bainite can be present in different forms.
The processing properties of the dual steel which determine the steel types such as a low yield ultimate ratio at very high tensile strength, a strong cold hardening and a good cold formability are well known.
Increasingly, multiphase steels are also used such as complex-phase steels, ferritic-bainitic steels, bainitic steels and also martensitic steels, which are characterized by different microstructure compositions as described in EN 10346.
Complex phase steels are steels which contain small proportions of martensite, residual austenite and/or perlite in a ferritic/bainitic basic structure wherein an extreme grain refinement is caused by a delayed re-crystallization or by precipitations of micro-alloy elements.
Ferritic bainitic steels are steels which contain bainite or strain hardened bainite in a matrix of ferrite and/or strain hardened ferrite. The hardening of the matrix is caused by a high dislocation density by grain refinement and the precipitation of micro alloy elements.
Bainitic steels are steels which are characterized by a very high yield strength and tensile strength at a sufficiently high expansion for cold forming processes. The chemical composition results in a good weldability. The microstructure typically consists of bainite. In some cases small proportions of other phases such as marteniste and ferrite can be contained.
Martensitic steels are steels, which as a result of thermo mechanical rolling contain small proportions of ferrite and/or bainite in a basic structure of martensite. The steel type is characterized by a very high yield strength and tensile strength at sufficiently high expansion for cold forming processes. Within the group of multi-phase steels the martenisititc steels have the highest tensile strength values.
These steels are used in structural components, chassis and crash-relevant components as well as flexibly cold rolled strips. This Tailor Rolled Blank lightweight construction technology (TRB®) enables a significant weight reduction as a result of the load adjusted selection of sheet thickness over the length of the component.
However, when strongly varying sheet thicknesses are involved, the alloys and continuous annealing systems known and available today impose certain limitations on the production of TRB®s with multiphase microstructure, for example with regard to heat treatment prior to the cold rolling. In regions of different sheet thickness, i.e., when varying degrees of rolling reduction are present, a homogenous multiphase microstructure cannot be established in cold rolled and hot rolled steel strips due to the temperature difference in the conventional process windows.
For economic reasons cold rolled steel strips are usually subjected to recrystallizing annealing in the continuous annealing process to generate well formable steel sheet. Depending on the alloy composition and the strip cross section, the process parameters such as throughput speed, annealing temperature and cooling rate, are adjusted corresponding to the mechanical-technological properties by way of the microstructure required therefore.
For establishing the dual-phase microstructure, the hot strip in typical thicknesses between 1.50 mm to 4.00 mm, or cold strip in typical thicknesses of 0.50 mm to 3.00 mm, is heated in the continuous annealing furnace to such a temperature that the required microstructure forms during the cooling. The same applies for configuring a steel with complex phase microstructure, martensitic, ferrite-bainitic and also purely bainitic microstructure.
In the continuous annealing system, a special heat treatment is applied in which relatively soft components such as ferrite or bainitic ferrite provide the steel with its low yield strength and hard components such as martensite or carbon-rich bainite provide it with its strength.
When high demands on corrosion protection require the surface of the hot or cold strip to be hot dip galvanized, the annealing is usually carried out in a continuous annealing furnace arranged upstream of the hot dip galvanizing bath.
In the continuous annealing of hot rolled or cold rolled steel strips with alloy concepts known for example from EP 1 113 085 A1, EP 1 201 780 A1 and EP 0 796 928 A1 for a multiphase steel involves the problem that with the there tested alloy compositions the demanded mechanical properties are satisfied but an only narrow process window is available for the annealing parameters in order to be able to ensure uniform mechanical properties over the strip length in the case of cross sectional steps without adjustment of the process parameters.
A further disadvantage of the steel known from EP 0 796 928 A1 is that the very high Al-contents of 0.4-2.5% adversely affects steel production via conventional band casting, due to micro segregation and casting powder inclusions.
In the case of widened process windows the required strip properties can also be achieved at same process parameters also in the case of greater cross sectional changes of the strips to be annealed.
Besides flexibly rolled strips, which have different thicknesses over their width, this applies in particular also to strips of different thicknesses and/or different widths, which have to be annealed subsequent to each other.
Especially in the case of different thicknesses in the transition region of one strip to another, a homogenous temperature distribution is difficult to achieve. In the case of alloy compositions with too narrow process windows this can lead to the fact that for example the thinner strip is either moved through the furnace too slowly, thereby lowering productivity, or that the thicker strip is moved through the furnace too fast and the required annealing temperature for achieving the desired microstructure is not reached. The result of this is more waste with the corresponding non-conformity costs.
The deciding process parameter is thus the adjustment of the speed in the continuous annealing because the phase transformation is temperature and time dependent. Thus, the less sensitive the steel is regarding the uniformity of the mechanical properties when temperature and time course change during the continuous annealing, the greater is the process window.
The problem of a too narrow process window is especially pronounced in the annealing treatment when stress-optimized components made of hot or cold strip are to be produced, which have sheet thicknesses that vary across the strip length and strip width (for example as a result of flexible rolling).
A method for producing a steel strip with different thickness across the strip length is for example described in DE 100 37 867 A1.
When using the known alloy concepts for the group of multiphase steels, the narrow process window makes it already difficult during the continuous annealing of strips with different thicknesses to establish uniform mechanical properties over the entire length of the strip.
In the case of flexibly rolled cold strip made of multiphase steels of known compositions, the too narrow process window either causes the regions with lower sheet thickness to have excessive strengths resulting from excessive martensite proportions due to the transformation processes during the cooling, or the regions with greater sheet thickness achieve insufficient strengths as a result of insufficient martensite proportions. Homogenous mechanical-technological properties across the strip length or width can practically not be achieved with the known alloy concepts in the continuous annealing.
The goal to achieve the resulting mechanical-technological properties in a narrow region across the strip width and strip length by the controlled adjustment of the volume proportions of the microstructure phases has highest priority and is therefore only possible through a widened process window. The known alloy concepts for multiphase steels are characterized by a too narrow process window and are therefore not suited for solving the present problem, in particular in the case of flexibly rolled strips. With the alloy concepts known to date only steels of a strength class with defined cross sectional regions (sheet thickness and strip width) can be produced, hence requiring different alloy concepts for different strength classes or cross sectional ranges.
The state of the art is to increase the strength by increasing the amount of carbon and/or silicone and/or manganese and via the microstructure adjustment and solid solution strengthening (solid solution hardening).
However, increasing the amounts of the aforementioned elements, increasingly worsens the material processing properties for example during welding, forming and hot dip coating.
On the other hand, there is also a trend in the steel production to reduce the carbon and/or manganese content in order to achieve a better cold processability and better performance properties.
For describing and quantifying the edge crack behavior, the hole expansion test according to ISO 11630 is used as one of multiple possible test methods. At corresponding optimized grades, the steel user expects higher values than in the standard material. However, increasingly the focus is also on welding suitability characterized by the carbon equivalent.
A low yield strength ratio (Re/Rm) is typical for a dual-phase steel and serves in particular for the formability in stretching and deep drawing processes. This provides the constructor with information regarding the distance between ensuing plastic deformation and failing of the material at quasi static load. Correspondingly lower yield strength ratios represent a greater safety margin for the component failure.
A higher yield strength ratio (Re/Rm) as it is typical for complex-phase steels is also characterized by a resistance against edge cracks. This can be attributed to the smaller differences in the strengths of the individual microstructure components, which has a positive effect on a homogenous deformation in the region of the cutting edge.
The analytical landscape for achieving multiphase steels with minimal strengths of 580 MPa has become more diverse and shows very broad alloy ranges regarding the strength-promoting elements carbon, silicone, manganese, phosphorous, aluminum and chromium and/or molybdenum as well as regarding the addition of micro-alloys such as titanium and vanadium and regarding the material characterizing properties.
The spectrum regarding dimensions is broad and lies in the thickness range of 0.50 to 4.00 mm. Predominantly strips up to about 1850 mm are used but also slit strip dimensions which are generated by longitudinally separating the strips. Sheets or plates are generated by transverse separation of the strips.